1. Field of the Invention
The present invention is in the field of protective coatings for structures made of corrodible metal, and the invention relates more particularly to a method for improving the corrosion resistance of metal structures that are covered by outer protective coatings such as resin coatings, including resin coatings reinforced with glass or other filler materials, employed for the protection of corrodible metal structures.
2. Description of the Prior Art
Despite the present advanced state of technology in the plastics industry, and the current widespread availability and use of a variety of resin protective coatings, including resin coatings that are reinforced with glass or other filler materials, the protection of metal bodies against corrosion remains a major industrial problem, and the worldwide expenditure in combating and replacing losses due to corrosion is currently one of the world's greatest economic losses. Many modern resin coatings will intrinsically provide an excellent barrier against various corrosive agents, including such atmospheric corrosive agents as oxygen, water vapor and carbon dioxide, as well as various man-generated atmospheric pollutants, and even against the severe corrosive agents of a marine environment such as those found in salt water. Nevertheless, even the best resin protective coatings, when applied according to current technology to large structures with extensive areas which are likely to be subjected to a severely corrosive environment, such as a marine environment, will not satisfactorily protect such structures from corrosion due to attack by both oxidation and electrolytic action. Examples of such large structures which are not adequately protectable by modern resin coatings applied according to current procedures and which therefore present a continuous problem of deterioration by corrosion when subjected to a highly corrosive environment such as a marine environment, are ship or boat hulls, offshore drilling or production platforms, bridges, pipelines, and the like. Examples of some other metal structures which involve corrosion problems on a very large scale although they are not necessarily subjected to a marine environment, are metal building structural members and panels, cargo shipping containers used on ships, trains, trucks and aircraft, the bodies or shells of various vehicles such as automobiles, trucks, buses, trains, aircraft and the like, and a variety of materials storage containers.
As an example of the severity of this corrosion problem in a marine environment, the present life expectancy of aluminum ship hulls in salt water is only approximately seven to ten years, even with attempts to protect them from corrosion by the use of the most modern resin coatings.
The conventional procedure for appying a resin protective coating, which may or may not be reinforced with glass or other filler material, onto a metal structure, is to first clean the structure, which may be done by sandblasting, and then to directly apply the resin coating over the cleaned structure. The nature of such resins is that they are characteristically too viscous to substantially penetrate into the pores of the metal surface, and hence are unable to displace moist air or other corrosive agents therefrom. Corrosive agents are therefore inevitably encapsulated in the pores underneath the coating and are free to immediately initiate and perpetuate corrosion from underneath the resin coating. All such entrapped air has a water vapor content, and substantial temperature reductions will cause at least a portion of such water vapor to precipitate as liquid water in the pores. Such moisture, in combination with carbon dioxide and other corrosive agents likely to be present in the air, are free to attack the edges of the interface between the outer surface of the metal and the resin covering at the pores, thereby undermining the bond between the resin covering and the metal surface, and this will occur no matter how well or by what means the outer surface of the metal structure was cleaned. It has historically been a matter of primary concern in the application of resin coatings to metal structures that the structures be entirely free of oil prior to application of the coating.
Such corrosive undermining of conventionally applied resin coatings on metal structures will proceed to occur from the time the coating was applied, at a rate that will depend upon the nature and extent of the corrosive agents captured within the pores, and this will ultimately result in blistering, separation and cracking of the resin coating. The corrosive undermining will be accelerated in areas underneath the resin coating adjacent any regions where the resin has been scratched away to expose bare metal to the environment.
One method that has heretofore been successfully employed to protect resin-coated metal objects from such corrosive undermining has been vacuum impregnation of the pores with resin. However, this method is only applicable to very small metal parts which are capable of being enclosed in a vacuum chamber to which high vacuum may be applied, and the method is not applicable to large metal structures having extended surface areas. Also, this vacuum impregnation method is critical in application, and is slow, time-consuming and expensive.
As indicated above, conventional practice in the application of resin coatings to metal objects requires that the objects be absolutely free of oil, and if it was thought that any oil might be present on the object, such oil was required to be completely removed, generally by chemical means, prior to application of the resin coating. Thus, any suggestion that oil might deliberately be applied to a metal structure as a preparatory step prior to the application of a resin coating would be diametrically opposed to conventional thinking and practice. In fact, the prior art specifically teaches the provision of an oil undercoat for the purpose of preventing a resin outer coating from bonding to metal objects; i.e., the prior art teaches that oil prevents a resin coating from bonding to a metal body. Thus, U.S. Pat. No. 3,084,066 to Dunmire teaches that chain links for marine use be provided with a full oil undercoating beneath a resin covering to prevent adherence of the resin covering to the metal and to provide lubrication for minimizing wear of adjacent links against each other. Retention of the outer resin coating on the objects is only permitted by the specialized nature and small size of the chain links, which enables the outer covering to encapsulate both the oil film and the individual links in a closed loop configuration. Similarly, in U.S. Pat. No. 3,443,982 to Kjellmark, Jr., individual wire strands for an oil well sucker rod each have a full oil-based undercoat with a tubular resin outer jacket that is supported by closed loop encapsulation of the oil around the narrow wire.
An early attempt to utilize oil under an outer coating was disclosed in U.S. Pat. No. 663,381 to Kopp, wherein a metal surface was first completely covered with coal oil, and then immediately an oil base paint was applied over the oil, so that " . . . the oil combines with part of the paint and carries it into the interstices and the paint and oil tend to unite, securing a close adhesion to the surface." This combination of oil and oil-based paint had several inherent defects which rendered it generally ineffective for protection against moisture and other corrosive agents. First, since the oil assertedly became combined with the paint, and had to be combined in order to effect any bond of the paint to the metal after the metal surface was "completely and fully" covered with the oil, then the oil simply became a part of the oil base paint. This resulted in a partial thinning of the paint which lessened its effectiveness in bonding and drying, while nevertheless leaving the paint with the inherent defect, now well recognized in the art, that when it did dry, evaporation of solvents therefrom left it porous and pervious to corrosive agents of the atmosphere and marine environments. A further defect of the Kopp oil and paint combination was that upon drying, the paint that had been carried into the interstices or pores suffered the usual shrinkage of drying paint, which caused the paint within the interstices or pores to pull away from the walls, the resulting spaces applying a pressure differential across the porous paint layer to draw air and its corrosive agents into these spaces through the pores in the paint. In this manner, from the time the paint commenced to dry, moisture and other corrosive agents of the atmosphere were drawn into the pores and free to initiate corrosive undermining of the paint covering.
It is notable that all three of the prior art patents referred to above which taught the application of oil prior to an outer coating prescribed that the oil should completely cover the surface of the metal under the outer coating. There was no teaching or suggestion in this prior art that only restricted, selected portions of the metal might be provided with oil under an outer coating, or that there might be any benefit in such an arrangement, or how such might be effected. In particular, the prior art did not teach or suggest that inner surfaces only of the metal, within the pores, be covered with a first protective or sealing material such as oil which is excluded from the outer surface, and that the outer surface only of the metal be bonded with a second protective or sealing material such as resin in a continuous covering that also bridges over the pores and the said first material. Nor was there any teaching or suggestion in the prior art as to how surface oil might be completely removed from an oil covered and impregnated metal surface so as to admit of an intimate bond with an outer resin coating, while nevertheless leaving the pores of the metal substantially completely filled with oil in the regions of the pore orifices where the oil could seal and protect the otherwise exposed and vulnerable edges of the interface between the metal surface and the outer coating.